Embodiments of the invention relate generally to electromotive control, and, more particularly, to micro-electromechanical system (MEMS) based motor starter, such as may be used for controlling motor operation and protecting the motor from overload and/or fault conditions.
In the field of motor control, a conventional motor starter may consist of a contactor and a motor overload relay. The contactor is typically a three-pole switch, which is usually operated by a continuously energized solenoid coil. Since the contactor controls the operation of the motor, i.e., the starting and stopping, this device is generally rated for many thousands of operations.
The overload relay generally provides overload protection to the motor from overload conditions. Overload conditions can occur, for example, when equipment is operated in excess of normal full-load rating, e.g., when conductors carry current in excess of the applicable ampacity ratings. Overload conditions persisting for a sufficient length of time will damage or overheat the equipment. The terms “overload,” “overload protection” and “overload relay” are well-understood in the art. See, for example, National Electrical Manufacturers Association (NEMA) standard ICS2, which is herein incorporated by reference.
To protect a motor from faults requiring instantaneous protection (such as short circuit faults, ground faults), circuit breakers, e.g. instantaneous trip circuit breakers, are typically used. Additionally these circuit breakers may function as a manual disconnect switch (disconnect), which serve to isolate the motor during a maintenance operation.
Devices which combine the instantaneous protection of a circuit breaker as well as the motor starter functions in a single enclosure are known in the art as combination starters. However, the current-carrying components of instantaneous trip circuit breakers are constructed of heavy copper bars and large-sized tungsten contacts. For example, the copper bars/contacts may be over-designed to survive short circuit faults, however, during a short circuit fault the load may be in parallel with the short and thus such over-design has little effect on the level of short circuit current.
The large size of the components increases the size of the circuit breaker to the extent that such circuit breakers do not fit within certain standard Asian and European circuit breaker enclosures. Moreover, instantaneous trip breakers may include complicated and/or costly mechanical switches that use electromechanical release mechanisms.
As noted above, these conventional circuit breakers are large in size thereby necessitating use of a large force to activate the switching mechanism. Additionally, the switches of these circuit breakers generally operate at relatively slow speeds. Furthermore, these circuit breakers are burdensomely complex to build and thus expensive to fabricate. In addition, when contacts of the switching mechanism in conventional circuit breakers are physically separated, an arc is typically formed there between which continues to carry current until the arc is extinguished naturally. Moreover, energy associated with the arc leads to degradation of the contacts and/or can raise other undesirable conditions in certain types of environments, such as near a flammable gas or material.
As an alternative to slow electromechanical switches, relatively fast solid-state switches have been employed in high speed switching applications. As will be appreciated, these solid-state switches switch between a conducting state and a non-conducting state through controlled application of a voltage or bias. For example, by reverse biasing a solid-state switch, the switch may be transitioned into a non-conducting state. However, since solid-state switches do not create a physical gap between contacts when they are switched into a non-conducting state, they experience leakage current. Furthermore, due to internal resistances, when solid-state switches operate in a conducting state, they experience a voltage drop. Both the voltage drop and leakage current contribute to the generation of excess heat under normal operating circumstances, which may be detrimental to switch performance and life.
U.S. patent application Ser. No. 11/314,336 filed on Dec. 20, 2005, which is incorporated by reference in its entirety herein, describes high-speed micro-electromechanical system (MEMS) based switching devices including circuitry and techniques adapted to suppress arc formation between contacts of the micro-electromechanical system switch. The response time of this switching circuitry is in the order of micro-to-nano-seconds (e.g., faster than a conventional fuse or breaker).
In view of the foregoing considerations it would be desirable to provide a motor starter for performing fast current limiting, achieving low let-through current during fault conditions, e.g., substantially lower than may be achieved with conventional motor-protecting technology, such as current limiting fuses or circuit breakers. It would be further desirable to provide a combination motor starter adapted to provide various functionality, such as motor control, fault protection, and overload protection, in an efficiently integrated system.